What’s the biochemical etiology of a panic attack?

Answer by Carlo Carandang:

A panic attack is a severe manifestation of the fight or flight response, as illustrated in the following flowchart:
(flowchart reproduced with permission from AnxietyBoss.com)
So with a panic attack, it can be triggered by a change in your body, such as increased heart rate, palpitations, or chest pains, and this in turn leads to thoughts about the trigger, and you may think you are experiencing a heart attack.
This alarming thought that you may be having a heart attack in turn leads to fear and anxiety, which activates the amygdala, which is an important part of the fear circuit in your brain. The activated amygdala in turn activates the both the locus coeruleus (sympathetic nervous system) and the hypothalamus (hypothalamic-pituitary-adrenal axis, or HPA axis).
With the activation of the sympathetic nervous system from the amygdala and locus coeruleus, neural impulses directly innervate many organs in the body, such as the heart (increased heart rate and contraction), eyes (dilated pupils), blood vessels (dilate in skeletal muscles and constrict in digestive and reproductive system), and sweat glands (increased perspiration).
In addition to the neural response from the sympathetic nervous system directly innervating organs to produce panic attack symptoms, there is also a simultaneous hormonal response from the HPA axis. So the amygdala activates the hypothalamus, and the hypothalamus in turn secretes CRF (corticotropin releasing factor), which then travels to the pituitary. This in turn activates the pituitary gland to secrete ACTH (adrenocorticotropic hormone) into the bloodstream, and the ACTH travels to the adrenal glands located on top of each kidney. The ACTH activates the adrenal cortex (the outer part of the adrenal glands), and this in turn secretes cortisol into the bloodstream. Cortisol is a stress hormone that increases glucose production to provide fuel for the panic attack, and also suppresses immune functioning.
To add to the hormonal response, the sympathetic nervous system also activates the adrenal medulla directly (the inner part of the adrenal glands), and this leads to the secretion of adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream, further adding to the panic attack symptoms, such as increasing heart rate, dilating bronchioles in the lungs, increasing the rate of breathing, and stimulating skeletal muscle contraction.
As the panic attack progresses, you will breathe faster, and will start to hyperventilate. As your rate of breathing increases, your breaths become more shallow, and you resort to chest breathing instead of belly breathing (breathing using your diaphragm muscle). As your breaths become more shallow and rapid, you blow off more carbon dioxide (CO2) and breathe in less oxygen (O2). This leads to respiratory alkalosis and constriction of the blood vessels in your brain. When combined with less O2 in the bloodstream, this can lead to fainting, dizziness, tingling in your lips and extremities, and even muscular spasms or seizures.
The way to correct hyperventilation from a panic attack is to recycle that CO2 you are blowing out by breathing into a paper bag.

What's the biochemical etiology of a panic attack?